1. Field of the Invention
The present invention relates to a carbon nanofiber for a fuel cell electrode, a nanocomposite for a fuel cell, a method of preparing the carbon nanofiber and the nanocomposite, an electrode including the nanocomposite, and a fuel cell including an electrode having the nanocomposite.
2. Description of the Related Art
Recently, research has been conducted into fuel cells as power sources for vehicles and electronic devices due to increasing concerns regarding environmental problems in burning fossil fuels and exhaustion of natural resources.
Fuel cells are devices that convert energy stored in fuel to electrical energy through electrochemical reactions between the fuel and oxidizing gas. Fuel cells can be classified into solid oxide fuel cells that operate at 1000° C. using a solid oxide, molten carbonate fuel cells that operate at 500 to 700° C., phosphoric acid fuel cells that operate at about 200° C., alkali electrolyte fuel cells that operate at room temperature to about 100° C., polymer electrolyte membrane fuel cells, and the like.
Polymer electrolyte fuel cells include proton exchange membrane fuel cells (PEMFCs) that use hydrogen gas as a fuel, direct methanol fuel cells (DMFCs) that directly supply liquid methanol as a fuel to an anode. PEMFCs, which are regarded as providing next generation clean power that can replace power sources that rely on fossil fuel, have high power density and a high energy conversion rate. Further, PEMFCs can operate at room temperature and can be easily miniaturized and sealed so that they can be used for various applications, such as electric vehicles, domestic energy generating systems, mobile communications equipment, medical equipment, military and space equipment, and business equipment.
PEMFCs as power generating systems that generate direct current electricity from electrochemical reactions between hydrogen and oxygen, and include an anode, a cathode, and a proton exchange membrane between the anode and the cathode.
The proton exchange membrane is formed of a solid polymer such as Nafion that has high proton conductivity and that is impermeable to unreacted gases. Each of the anode and the cathode respectively includes a supporting layer to supply reactant gases or liquids and a catalyst layer in which oxidation/reduction of reactant gases occur.
In PEMFCs having the structure described above, hydrogen is supplied as the reactant gas to the anode where it is oxidized to convert hydrogen atoms into hydrogen ions and electrons. The hydrogen ions are conducted to the cathode across the proton exchange membrane.
Reduction reactions occur at the cathode where oxygen atoms receive electrons and are converted to oxygen ions which react with the hydrogen ions conducted from the anode across the proton exchange membrane to generate water.
Each of the anode and the cathode of the PEMFC includes a gas diffusion layer (GDL). Catalyst layers for promoting the chemical reactions in fuel cells are formed on supporting layers made of carbon cloth or carbon paper.
Direct Methanol Fuel Cells (DMFCs) having structures similar to PEMFCs supply liquid methanol as a reactant instead of hydrogen to the anode where the methanol is oxidized by a catalyst to produce hydrogen ions, electrons and carbon dioxide. While DMFCs have lower cell efficiency than PEMFCs, they can be more easily applied to portable electronic devices due to their use of a liquid fuel.
Research into electrodes, fuels, and electrolyte membranes has been actively conducted to improve power density and power output by increasing energy density in fuel cells. In particular, attempts have been made to improve activity of a catalyst used in the electrodes.
A metal such as Pt, Pd, Rh or Ru, or an alloy of Pt and other metals is generally used as the catalyst in PEMFCs and DMFCs. To be cost-effective, reduced amounts of the metal catalyst are desired.
Thus, as a method of reducing the amount of the catalyst while maintaining or improving the performance of fuel cells, a method of using a conductive carbon material having a large surface area as a support by dispersing Pt, or the like into micro particles to enlarge the surface area of the catalyst metal is used.
Typically, a catalyst formed of Pt is prepared in a paste form and uniformly coated on a porous carbon support. However, the degree of dispersion of the catalyst is not uniform in the carbon support.